1. Field of the Invention
The present invention relates to a fixing device used for an image forming apparatus such as a laser beam printer, a multi-functional peripheral or the like and an image forming apparatus using the same, and particularly, relates to a fixing device including fixing roller that fuses an unfixed toner to fix to a recording medium and an image forming apparatus using the same.
2. Description of the Prior Art
Conventionally, as a fixing device used for an electrophotographic image forming apparatus such as a multi-functional peripheral, a printer or the like, a fixing device employing a so-called heat-roller fixing type is widely used.
The fixing device employing the heat-roller fixing type includes a pair of rollers (a fixing roller and a pressing roller) in press-contact with each other, and after the pair of the rollers is heated to a predetermined temperature (fixing temperature) by a heating portion composed of a halogen heater and the like arranged inside both rollers of the pair of the rollers or either one of the rollers, a recording medium such as a recording sheet on which an unfixed toner image is formed is fed to a press-contact portion (fixing nip portion) of the pair of the rollers to pass through the press-contact portion so that the toner image is fixed on the recording sheet by heat and pressure.
Incidentally, in the fixing device provided in a color image forming apparatus, an elastic roller provided with an elastic layer made of silicone rubber and the like is generally used on a fixing roller surface layer in contact with an unfixed toner image.
The elastic roller is used for the fixing roller, and the fixing roller surface is thereby elastically deformed corresponding to unevenness of the unfixed toner image to come in contact with the surface of the toner image so as to cover, so that it becomes possible to perform favorable heating and fixation for an unfixed color toner image having a larger amount of the toner compared to that of an unfixed monochrome toner image.
Further, the color image forming apparatus using the elastic roller as the fixing roller is able to improve a strain release properties with respect to a color toner which is easily subjected to offset compared to a monochrome toner due to a strain release effect of the elastic layer of a fixing nip portion.
Moreover, the color image forming apparatus has the fixing nip portion whose nip shape becomes a protruded shape toward the fixing roller side (what is called, an inverse nip shape), which makes it possible to improve peeling performance of a recording sheet, and to allow the recording sheet to be peeled off without using a peeling portion such as a peeling claw (self-stripping), so that it is possible to remove an image defect caused by the peeling portion.
Additionally, in order to form a color image forming apparatus corresponding to recent higher processing speed, the fixing device provided in the color image forming apparatus needs to be configured so as to form a fixing nip portion having a wide nip width. Methods of widening the nip width of the fixing nip portion include a method of thickening a layer thickness of the elastic layer of the fixing roller, a method of enlarging a fixing roller diameter and the like.
However, since the elastic layer provided in the fixing roller has very low heat conductivity, a problem arises such that a surface temperature of the fixing roller does not move on with increase in the processing speed in a case where the heating portion is inside the fixing roller.
On the other hand, a problem arises such that a larger fixing roller diameter requires a longer warming-up time until the surface temperature of the fixing roller reaches the fixing temperature and increased power consumption.
In order to solve the above-described conventional problems, a belt fixing type fixing device using a fixing belt is disclosed as a fixing device provided in a color image forming apparatus (refer to Patent Literature 1).
The belt fixing type fixing device has no heating portions provided in a fixing roller and a pressing roller; includes a heating roller separately from the fixing roller and the pressing roller; has a fixing belt suspended between the heating roller and the fixing roller; has the pressing roller facing the fixing roller via the fixing belt; and is configured such that the fixing belt is heated with a heat source arranged inside the heating roller.
In the belt fixing type fixing device, a fixing nip portion is formed in a press-contact part where the fixing roller comes into press-contact with the pressing roller via the fixing belt. Such a belt fixing type fixing device is configured so that the fixing belt whose heat capacity is small is heated, and it is thus possible to shorten the warming-up time until the surface temperature of the fixing belt reaches the fixing temperature.
Further, the belt fixing type fixing device does not need to arrange a heating portion such as a halogen heater inside the fixing roller, and the fixing roller provided with a thick elastic layer with low hardness made of sponge rubber or the like is able to be used, so that it is possible to form the fixing nip portion with a wide nip width.
In this manner, the belt fixing type fixing device is, since boot speed is fast, allowed to have no preheating during ready and waiting as well as to shorten a waiting time, and is provided in an image forming apparatus of not only a miniature low-speed machine but also a large machine and a high-speed machine.
Next, description will be given for cleaning of the fixing roller of the above-described fixing device.
First, description will be given in detail for an example of a conventional fixing device that does not use a fixing belt with reference to the drawings.
FIG. 1 is an illustrative diagram showing a configuration of the conventional fixing device that does not use the fixing belt.
A conventional fixing device 150 is provided with, as shown in FIG. 1, for example, a cleaning device 160 using a cleaning web 161 (hereinafter, simply referred to as “web”) in addition to a fixing roller 151 and a pressing roller 152.
The fixing roller 151 is provided with heating portions 151a and 151b. The pressing roller 152 is provided with a heating portion 152a. The fixing roller 151 and the pressing roller 152 which are configured in this manner are brought into press-contact with each other so that a fixing nip 153 is formed. A recording sheet 32 on which an unfixed toner image T0 is formed passes through the fixing nip 153 between the fixing roller 151 and the pressing roller 152, thereby fusing an unfixed toner to be fixed onto the recording sheet 32.
The cleaning device 160 is provided with a web 161, a delivery roller 162, a wind-up roller 163 and a press-contact roller 164.
The web 161 is impregnated with anti-toner-adhesive oil, and comes into contact with the surface of the fixing roller 151, thereby adsorbing and removing a toner that adheres to the surface of the fixing roller 151 as well as applying the anti-toner-adhesive oil to the surface of the fixing roller 151.
The delivery roller 162 is rotatably provided to deliver the web 161. The wind-up roller 163 is rotatably provided to be delivered from the delivery roller 162 and rewinds the cleaning web 161 after coming into contact with the fixing roller 151.
The press-contact roller 164 is rotatably provided and brings the web 161 into press-contact with the surface of the fixing roller 151. That is, the web 161 is held between the fixing roller 151 and the press-contact roller 164. A press-contact portion between the fixing roller 151 and the web 161 thus becomes a cleaning nip portion 165.
In the conventional fixing device 150, the web 161 is delivered from the delivery roller 162, passes through the cleaning nip portion 165, and is rewound by the wind-up roller 163. Subsequently, the web 161, at the time of passing through the cleaning nip portion 165, removes a toner that adheres to the surface of the fixing roller 151 as well as applies the oil to the surface of the fixing roller 151.
In this manner, the fixing device 150 becomes a favorable state since the surface of the fixing roller 151 is cleaned up by the cleaning device 160.
Next, description will be given for a conventional belt fixing type fixing device using a fixing belt.
In a fixing device using a fixing belt with low heat capacity, since heat storage performance from a heater to the fixing belt are smaller compared to a heat quantity that shifts from the fixing belt to the recording sheet, when the surface temperature of the fixing belt and the pressing roller is lowered and is not maintained in an appropriate temperature range, an offset amount (amount to be transferred to the fixing belt) of the toner is increased more than usual, and a problem that the fixing belt is contaminated easily occurs. Such a problem tends to occur notably when an image with a high halftone print rate is printed in large quantity.
Additionally, when image formation is performed using the above-described belt fixing method, the surface of the fixing belt is charged by combining the fixing belt and the pressing roller, and a phenomenon occurs such that the surface of the fixing belt attracts the toner from the recording sheet (what is called, electrostatic offset) in some cases.
When the electrostatic offset occurs, the toner is retransferred onto the recording sheet and comes up to the surface as a remained image after the fixing belt goes around, or a large amount of the toner adheres to a cleaning member (for example, cleaning roller) that is brought into contact with the fixing belt in order to prevent such a case, and the toner that once adheres to the cleaning member is thereby fused (melted) again to adhere to the fixing belt, then a phenomenon that contaminates the recording sheet occurs in some cases.
Moreover, there is a problem that since the temperature of the pressing roller is lower than the temperature of the fixing belt, the toner that adheres to the fixing belt (offset toner) moves to the surface of the pressing roller and adheres to the peeling claw to cause image contamination on the recording sheet.
In this manner, there is a case where a toner or an offset toner of an unfixed toner image is adhered to the surface of the fixing belt. The toner that adheres to the fixing belt is fixed to another recording sheet and recognized as image contamination or the like. Therefore, various measures are employed in order to prevent the toner from adhering to the surface of the fixing belt or to remove the toner that adheres to the surface of the fixing belt.
For example, a fixing device, in order to remove the offset toner, brings the pressing roller into contact with a roller applied with a slight amount of oil, and brings the roller applied with oil into contact with the cleaning roller that cleans up the surface thereof is disclosed (refer to Patent Literature 2).
As another example, a fixing device is disclosed that, in an image heating device that includes a heating member, a pressing member brought into press-contact therewith and a cleaning member arranged facing the pressing member, and performs heat-processing of a toner image by guiding a recorded material that carries the toner image to be held and conveyed by a press-contact nip portion formed with the heating member and the pressing member, includes a cleaning member contact/separation portion that, in order to clean up the offset toner caused by lowering of the temperature of the pressing roller, uses a metallic roller whose surface energy is higher than that of the pressing roller, and brings the cleaning member into contact with/separation from the pressing member corresponding to a type of the recorded member (refer to Patent Literature 3).
Further, a fixing device is disclosed that includes a fixing member arranged on the side of a toner image of a recording medium that carries the toner image and a pressing member arranged on the side of a non-toner image of the recording medium, and fixes the toner image to the recording medium by heating and pressing the toner of the toner image when the recording medium that carries the toner image passes through between the fixing member and the pressing member, and includes a variable mechanism that varies the distance between the fixing member and the pressing member, and a cleaning member that comes into contact with the pressing member when the fixing member is placed at a position where the fixing member separates from the pressing member to clean up the pressing member (refer to Patent Literature 4).